How did the notorious backside get from Iraq to Britain, where it
will be auctioned?

AFP explains:

A former soldier from Britain’s elite SAS regiment retrieved the
two-foot (0.6-meter) wide piece of history and took it back to
Britain shortly after U.S. marines dragged the statue down on
live television.

Nigel “Spud” Ely, now 52, was working with media covering the
fall of Baghdad at the time. He said the marines gave him
permission to remove the buttock using a hammer and a crowbar.

The resulting chunk is about two square feet, and is expected to
sell for around $15,400 at the Oct. 27 sale by Hansons
Auctioneers in Derby, England.